Mast cells are key effector cells in a wide variety of physiological and pathological processes, including innate immune responses and allergic disorders, chronic inflammatory diseases such as cardiovascular disease and arthritis, and tumor progression. Activated mast cells secrete a diverse array of factors that mediate their roles in inflammation, immunity, and tissue remodeling. However the exact mechanisms through which various genetic factors influence multiple aspects of mast cell biology have yet to be fully defined. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression and their dysregulation is implicated in numerous pathologic conditions. Recent work by our laboratory found that miR-9 over-expression was associated with aggressive, metastatic canine mast cell tumors (MCT), a well-established model of spontaneous malignant mast cell disease. Furthermore, enforced high expression of miR-9 in normal and malignant mouse mast cells with low basal levels of this miR using lentiviral vectors promoted invasion and enhanced the expression of CMA1, a mast cell-specific protease involved in tissue remodeling. To better study the role of miR9 in mast cell biology, we have generated a transgenic model of tissue specific induced miR-9 expression, have crossed these mice with the carboxypeptidase-3-Cre transgenic mice generated by our collaborator Dr. Stephen Galli (Stanford University), and confirmed the functionality of this model including enhanced invasive properties of mast cells from the CPA3-Cre/miR-9 double transgenics. We therefore hypothesize that miR-9 enhances CMA1 expression and promotes invasion in mast cells through the modulation of factors responsible for tissue remodeling and angiogenesis. We further hypothesize that in normal mast cells, miR-9 alters cell motility and enhances sensitivity to antigen stimulation in vivo, and thatin mouse models of mastocytosis, miR-9 contributes to enhanced cell invasion and angiogenesis and is associated with a more aggressive biological behavior. To test these hypotheses, we will complete the following specific aims: 1) Identify the molecular mechanisms responsible for miR-9-induced CMA1 expression and enhanced invasion in normal mast cells; 2) Assess the effects of miR-9 on normal mast cell biology in vivo utilizing our transgenic mouse model of tissue specific miR-9 expression; and 3) Investigate the contribution of miR-9 in promoting mast cell invasion, metastasis, and tumor progression in mouse models of malignant mast cell disease. In summary, the studies outlined in this K01 proposal will provide a more complete understanding of the molecular mechanisms through which miR-9 regulates mast cell behavior both in vitro and in vivo, particularly as it relates to induction of the metastatic phenotype. The Candidate: Dr Joelle Fenger, a licensed veterinarian and Board Certified Veterinary Medical Oncologist, is currently completing her PhD in the Graduate Studies Program in Comparative and Veterinary Medicine at The Ohio State University in the context of a dual Residency/PhD program. During the K01 SERCA award period, Dr. Fenger's efforts will first be dedicated to completing her PhD thesis studies and completing her dissertation. Over the final years, she will be appointed as a Research Assistant Professor during which time she will gain further skill sets and develop a refined research focus that will serve as the foundation for her transition to an independent academic scientist in the final year of the award. The Environment: Dr. Fenger's co-mentors, Dr. Cheryl London and Dr. Guido Marcucci are both Professors and Principal Investigators at OSU; Dr. London in the Department of Veterinary Biosciences, College of Veterinary Medicine, and Dr. Marcucci in the Department of Internal Medicine, College of Medicine. Dr. London has extensive experience in the biology of malignant mast cell disease and translational oncology, and Dr. Marcucci has substantial expertise in the role of miRNA in hematologic malignancies and preclinical therapeutic targeting of miRNAs. As part of the overall development and training plan during the period of the award, Dr. Fenger will receiving mentorship from a diverse, inter-disciplinary group of advisors and collaborators. She will develop skill sets in the broader areas of miRNA dysregulation in disease, receive formal laboratory training in mast-cell specific techniques, and advanced training in mouse pathobiology and mouse models of disease. Additionally, throughout the duration of the award, Dr. Fenger will be mentored in executing hypothesis-driven research and establishing collaborative research partnerships that will prepare her as she transitions to a career as an independent researcher.